Semiconductor memory devices may be classified as volatile memory devices and non-volatile memory devices. Non-volatile memory devices may maintain the data stored therein even when power is cut off. Accordingly, non-volatile memory devices may be employed in computers, mobile telecommunication systems, memory cards, etc.
A flash memory device is one such non-volatile memory device. Memory cells having a stacked gate structure may be used in flash memory devices. The stacked gate structure may include a tunnel oxide layer, a floating gate, an inter-gate dielectric layer, and a control gate electrode, which may be sequentially stacked on a channel region. In order to increase reliability and/or program efficiency of the flash memory cell, the film quality of the tunnel oxide layer may be improved, and/or a coupling ratio of the cell may be increased.
Resistive memory devices have also been developed. Resistive memory devices may include magnetic random access memory (MRAM) devices, phase change memory devices, and/or resistive random access memory (RRAM) devices. A unit cell of a resistive memory device may include a data storage element having two electrodes and a variable resistor layer that is interposed therebetween. When the resistive memory device is a MRAM device, the variable resistor layer may include a pinned layer, a tunneling insulating layer, and a free layer, which may be sequentially stacked, and the tunneling insulating layer and the free layer may include a ferromagnetic layer. When the resistive memory device is a phase change memory device, the variable resistor layer may include a phase change material layer, such as a chalcogenide layer. When the resistive memory device is a RRAM device, the variable resistor layer may be a praseodymium calcium manganese oxide ((Pr,Ca)MnO3) layer (“PCMO layer”). The variable resistor layer, i.e., the data storage material layer, may have a first resistance or a second resistance that is higher than the first resistance depending on polarity and/or magnitude of an electrical signal (voltage or current) that is applied between the electrodes.
For example, a RRAM device is disclosed in Korean Laid-Open Patent Publication No. 10-2004-79328 entitled “Non-Volatile Semiconductor Memory Device”. According to the Korean Laid-Open Patent Publication, a plurality of NAND-type cell units may be connected in parallel to one bit line, and each of the NAND-type cell units may include a plurality of variable resistive devices that are serially connected to each other and a plurality of switching MOS transistors that are serially connected to each other. Each of the switching MOS transistors may be connected in parallel to one of the variable resistive devices. The switching MOS transistors may be one-dimensionally disposed on a semiconductor substrate, and the variable resistive devices may be provided on the MOS transistors. Therefore, improvements in device integration in conventional RRAM devices that use NAND-type cell units may be limited.
As another example, a phase change memory device is disclosed in Japanese Laid-Open Patent Publication No. 2005-260014 entitled “Semiconductor Device”. According to the Japanese Laid-Open Patent Publication, a pair of phase change memory cells may be stacked on a semiconductor substrate, and a bit line may be interposed between the pair of phase change memory cells. That is, the pair of stacked phase change memory cells may share one bit line that is interposed therebetween. Consequently, the Japanese Laid-Open Patent Publication discloses a NOR-type phase change memory device.